In the internal combustion engines used for automobiles, motorcycles, agricultural equipment, machine tools, marine equipment, and the like powered with gasoline, light oil, or the like, in order to improve fuel economy or output, injectors that directly inject fuel into cylinders have been conventionally used. These injectors are designated as “cylinder injection direct injector” or “direct injector (DI).”
An engine using a cylinder injection direct injector is required to use fuel pressurized to high pressure unlike a conventional indirect injector in which a fuel is injected into an intake passage or an intake port to form air-fuel mixture. In the engine, therefore, high energy (voltage) is required for valve opening operation of the injector. To enhance controllability of the direct injector and achieve high-speed driving, it is required to supply the injector with high energy in a short time.
Many of conventional internal combustion engine controllers for controlling the direct injectors of internal combustion engines have boost circuits for boosting the voltage of battery as power supply to boost electric power supplied to the injectors.
FIG. 8 is a circuit diagram illustrating a conventional internal combustion engine controller. As illustrated in FIG. 8, the internal combustion engine controller includes a boost circuit 100 that is placed between a drive circuit 2 for driving a direct injector (DI) 3 and a battery 1 as power supply. The boost circuit boosts battery-power supply voltage to a higher voltage in a short time and supplies this boost voltage V100 to the drive circuit 2. The boost circuit 100 includes: a booster coil 110 that boosts the voltage (power supply voltage) of the battery; a switch element 120 that turns on/off power application to the booster coil 110; and a booster capacitor 130 that is inserted in parallel with the switch element 120 through a charging diode 140 for backflow prevention and stores energy from the booster coil 110. The switch element 120 is connected with a booster control circuit 150 that controls turn-on/off of the switch element 120. The booster control circuit 150 includes: a boost control part 151 that controls driving of the switch element 120; a voltage sensor part 152 that senses a charging voltage of the booster capacitor 130; and a current sensor part 153 that senses a current passed through the switch element 120. As the result of control by the boost control part 151, when the switch element 120 is turned on, a current from the battery 1 flows to the booster coil 110 through the switch element 120 and electrical energy is stored in the booster coil 110 by the inductance of the coil. When the switch element 120 is turned off, the current having passed through the booster coil 110 is interrupted and the booster capacitor 130 is charged with electrical energy of the booster coil 110.
FIG. 3(e) is an example of a current waveform of injector energization current 3A passed through the direct injector 3. As indicated by FIG. 3(e), in an initial stage of the passage of current through the injector 3, the injector energization current 3A is increased up to a predetermined upper limit peak current 460 in a short time by boost voltage 100A (peak current passing period 463). This peak current value is to open a valve of the injector 3 and larger by 5 to 20 times or so than the peak current value of injector energization current passed through conventional indirect type injectors.
After the end of the peak current passing period 463, the electric power supplied to the injector 3 is changed from boost voltage 100A to a voltage of the battery 1, and the current supplied to the injector 3 is controlled to a first hold current 461-1 to 461-2 as a current that is ½ to ⅓ or so of the peak current (a hold current is to hold a valve opening of the injector). Thereafter, the current is controlled to a second hold current 462 as a current that is ⅔ to ½ of the first hold current. During periods of the passage of the peak current 460, the first and second hold currents, the injector 3 is opened and injects fuel into the cylinder.
The process of changing from the upper limit peak current 460 to the first hold current is determined by the following elements: the magnetic circuit characteristic and fuel spray characteristic of the injector 3; the injector energization current passing period corresponding to a fuel supply quantity determined by the fuel pressure of a common rail for supplying fuel to the injector 3 and power requested of the internal combustion engine; and the like. The process includes those in the following cases: cases where the current is stepped down in a short time; cases where the current is gently stepped down; cases where the current is gently stepped down during a peak current gentle step-down period 464-1 and is stepped down in a short time during a peak current steep step-down period 464-2 as indicated by FIG. 3(e); and the like.
In order to quickly close the injector 3 after the end of fuel injection, the internal combustion engine controller is required to shorten the passage of current for a step-down period 466 of the injector energization current 3A (namely, a period for which the injector energization current 3A is stepped down from the second hold current 462 to a ground level) to interrupt the injector energization current 3A. Further, it is also required to step-down the injector energization current 3A in short time in the process 464-2 of stepping down the current from the peak current 460 to the first hold current 461-1, and in the process 465 of stepping down the current from the first hold current 461-2 to the second hold current 462.
However, since the injector energization current 3A is being passed through the driving coil of the injector 3 and high energy arising from the inductance of the coil is stored, in order to step down the injector energization current 3 in short time, it is required to eliminate such stored energy from the injector 3. There are some methods to achieve the elimination of the stored energy of the injector driving coil in the short step-down period 466. Such methods include: a method of utilizing the Zener diode effect in a drive element of the drive circuit 2 forming the injector energization current 3A to convert supplied energy into thermal energy; a method of regenerating the energy to the booster capacitor 130 for the driving energy of the injector driving coil through a current regenerating diode 5 placed between the drive circuit 2 and the boost circuit 100; and the like.
The above method of converting the energy into thermal energy makes it possible to simplify the drive circuit 2. However, converting the energy of an injector 3 into thermal energy is unsuitable for drive circuits involving the passage of large current.
Meanwhile, the above method of regenerating the energy to the booster capacitor 130 makes it possible to relatively suppress heating from the drive circuit 2 even when a large current is passed through an injector 3. Therefore, the method is widely used, especially, in engines in which a large current is passed through an injector 3. Such engines include engines using a direct injector that uses light oil (these engines are also designated as “common rail engines” sometimes); engines using a direct injector powered with gasoline; and the like.
An example of the controllers using a boost circuit that regenerates the stored energy of an injector driving coil to a booster capacitor is disclosed in Patent Document JP-A-2001-55948. Description will be given to the operation of this boost circuit with reference to FIG. 8 and FIG. 3.
The drive circuit 2 uses the boost voltage 100A of the boost circuit 100 to pass the injector energization current 3A through the injector 3. As a result, it is detected by the voltage sensor part 152 that the boost voltage 100A has dropped to a voltage 401 as a reference for starting a boost operation or below, as indicated by FIG. 3(a), the boost control part 151 starts the boost operation (incidentally, in FIG. 3(a), a reference numeral 400 denotes 0 [V]). The boost control part 151 changes a boost control signal 151B for the passage of current through the switch element 120 from LOW to HIGH. As a result, the switch element 120 is turned on, and a current flows from the battery 1 to the booster coil 110 and energy is stored in the booster coil 110. The booster coil current 110A passing through the booster coil 110 is converted into a voltage by a current sensing resistor 160 as the voltage for indicating a current passing through the switching element 120 (hereafter, referred to as “switching current for boosting”) 160A. It is then detected by the current sensor part 153. When the waveform of the switching current 160A for boosting detected at the current sensor part 153 is as indicated by FIG. 3(b). When the switching current 160A for boosting exceeds a preset switching stop threshold value 410 as indicated by FIG. 3(b), the boost control part 151 changes the boost control signal 151B for controlling the switch element 120 from HIGH to LOW to interrupt the switching current 160A. As the result of this interruption, the current having passed through the booster coil 110 cannot flow to ground 4 through the switch element 120 anymore. The energy stored by the inductance of the booster coil 110 generates high-voltage. When the voltage of the booster coil 110 becomes higher than the voltage obtained by the boost voltage 100A accumulated in the booster capacitor 130 and the forward voltage of the charging diode 140, the energy stored in the booster coil 110 migrates as a charging current 140A to the booster capacitor 130 through the charging diode 140. As indicated by FIG. 3(d), an initial value of the charging current 140A is a level of the current passing through the booster coil 110 immediately before the switch element 120 is interrupted, namely, the level of the switching stop threshold value 410, and then the charging current 140A decreases rapidly.
When it is detected that the boost voltage 100A boosted by the above operation does not reach the reference voltage 402 of a predetermined boost stop level, the boost control part 151 changes the boost control signal 151B from LOW to HIGH according to a boost switching cycle to pass current through the switch element 120 without detection of charging current 140A. This operation is repeated until the boost voltage reaches the voltage 402 of the predetermined boost stop level (boost voltage recovery time 403).
Meanwhile, when interruption or step-down in a short time of the injector energization current 3A is started by the drive circuit 2, a regenerative current from the injector 3 flows into the booster capacitor 130 through the current regenerating diode 5 during the step-down period 466 of the second hold current, the step-down period 464-2 of the peak current, and the step-down period 465 of the first hold current. Thus, similarly with boost operation by the booster coil 110, the energy stored in the inductance of the injector 3 migrates to the booster capacitor 130 and the boost voltage 100A is boosted.
As mentioned above, the boost circuit 100 detects the switching current 160A for boosting and carries out control so that the switching current 160A does not exceed over the switching stop threshold value 410. The boost circuit 100 can hold down the switching current 160A for boosting as compared with boost circuits that carries out control according to a predetermined time without detecting the switching current 160A for boosting (Refer to Patent Document JP-A-9-285108, and JP-A-2004-346808 for example.) Therefore, the boost circuit 100 makes it possible to minimize heating from the switch element 120, booster coil 110, and charging diode 140.
FIG. 5 illustrates a correlation between a boost voltage recovery time 403 and a battery voltage Vbat. As illustrated in FIG. 5, the boost voltage recovery time 403 does not vary depending on the battery-power supply voltage Vbat within a characteristic guaranteed battery voltage range (normal VB) 519 equal to or higher than a characteristic guaranteed minimum battery power supply voltage 516 and an operable high battery voltage range (high VB) 520 equal to or higher than an operable high battery power supply voltage 517. The reason for this is as follows: when the battery voltage is equal to or higher than the characteristic guaranteed minimum battery power supply voltage 516, the switching current 160A for boosting reaches the switching stop threshold value 410 in the predetermined boost switching cycle; and a period required for charging the energy stored in the booster coil 110 into the booster capacitor 130 is within a period behind the stop of switching in the boost switching cycle. The switching stop threshold value 410 is a value so adjusted that a normal-voltage boost voltage recovery request time 513 can be met at the characteristic guaranteed minimum battery power supply voltage 516. This request time 513 is a minimum required boost voltage recovery time requested of the boost circuit 100 by the drive circuit 2 to open an injector 3 in a predetermined time (at predetermined intervals) when the battery power supply voltage is normal voltage. Therefore, energy charged to the booster capacitor 130 by one time of boost switching operation is constant. Within a range equal to or higher than the characteristic guaranteed minimum battery power supply voltage 516, the boost voltage recovery time 403 is equal to or lower than the normal-voltage boost voltage recovery request time 513.
However, when the battery voltage Vbat drops into an operable low battery voltage range (low VB) 518 lower than the characteristic guaranteed minimum battery voltage 516, as illustrated in FIG. 4B, the switching current 160A for boosting does not reach the switching stop threshold value 410 within a predetermined boost switching cycle 500. Therefore, the period required to charge the energy stored the booster coil 110 into the booster capacitor 130 (booster coil charging period 502′) is shifted to the next boost switching cycle 500. Consequently, the period from the end of the booster coil charging period to the start of the next switching cycle 500, namely the period during which the booster coil current 110A is not energized (boost operation stop period 503) is lengthened. Therefore, the boost voltage recovery time 403 is lengthened by the influence of the battery voltage Vbat drop. As a result, the low-voltage boost voltage recovery request time 512 in FIG. 5 may not be met sometimes. This request time 512 is a minimum required boost voltage recovery time, which is requested to the boost circuit by the drive circuit 2 to open a valve of the injector in a predetermined time (at predetermined intervals) when the battery voltage is equal to or lower than the characteristic guaranteed minimum battery voltage 516.
The present invention is to provide an internal combustion engine controller that makes it possible to minimize the lengthening of the boost voltage recovery time of a boost circuit when battery voltage drops and to meet a low-voltage boost voltage recovery request time to solve the above problem.